1. Field of the Invention
The invention relates to a sheet forming system for a machine for producing a multilayer fibrous web, especially a multilayer paper or cardboard web consisting of at least two fibrous stock suspensions, including a multilayer headbox with a headbox nozzle which comprises at least two converging nozzle chambers extending over the width of the machine and separated from one another by at least one separating wedge, each carrying one fibrous stock suspension in the form of a fibrous suspension stream during operation of the multilayer headbox. Said nozzle chambers are respectively equipped with an upstream feed device and a downstream outlet gap respectively extending over the width and comprising a gap width, the two outer nozzle chambers respectively having an outside wall on their outside and the separating wedge having two separating wedge surfaces which are in contact with the respectively adjacent fibrous stock suspension streams during operation of the multilayer headbox; and one gap former which is located immediately downstream from the multilayer headbox and which has two continuous revolving wires, the first wire of which runs over a circumferential area of a forming roll and the second wire of which runs over a circumferential area of a breast roll and then runs onto the first wire in the area of the forming roll, thus forming a wedge shaped stock inlet gap which immediately receives the fibrous stock suspension streams emerging from the headbox nozzle of the multilayer headbox as one common fibrous stock suspension open jet, having a free jet length and the two wires with the at least two fibrous stock suspensions between them form a twin wire zone at least in sections.
2. Description of the Related Art
The fibrous stock suspensions are, as a rule, suspensions of different fiber stocks; they can however also be suspensions of same fiber stocks, whereby however different physical properties prevail. A physical property can for example be varying pressures for adjustment of different flow speeds in the respective fibrous stock suspension stream.
A multilayer headbox of this type for a machine to produce a multilayer fibrous web, especially a multilayer paper or cardboard web from at least two fibrous stock suspension is known for example from German disclosure document DE 195 38 149 A1. In the multilayer headbox in the embodiment of a three-layer headbox the suspension layers are kept separated by rigid separation walls. At the downstream end adjacent separation walls can be arranged differently vertical to the plane orientation of the jet so that good jet control is possible, as well as optimum combining of the suspension layers after discharge from the headbox.
Moreover, a multilayer headbox of this type is also known from the German patent document DE 43 23 050 C1. As shown in FIG. 3 of said German patent document, the disclosed multilayer headbox has a continuously tapering separating wedge for separation of two adjacent fibrous stock suspension streams in the headbox nozzle which is arranged movable by means of a linked element in upstream direction in the headbox nozzle.
In addition, a gap former of this type for a machine to produce a multilayer fibrous web, especially a multilayer paper or cardboard web from at least two fibrous stock suspensions is known for example from the German disclosure document DE 10 2005 003 531 A1. Especially FIGS. 1A through 4B respectively show a gap former whose first turning element for the first wire is a forming roll and whose second turning element for the second wire is a breast roll.
The majority of multilayer headboxes are currently employed in the packaging machinery field for the production of test liner. The increasing production speeds, as well as increasing raw material and energy costs increasingly demand the production of multilayer products with lower base weights. To this end the multilayer headboxes are operated with ever smaller gap widths or respectively fibrous stock suspension jet thicknesses. However, this increases the demands upon stability and turbulence quality of a respective fibrous stock suspension stream emerging from the headbox nozzle of the multilayer headbox for the purpose of reducing the mixing zones within the fibrous stock suspension stream in its height direction. The height direction is also referred to as “z-direction” by experts.
It is therefore the objective of the invention, and what is needed in the art is, to improve a sheet forming system of the type referred to at the beginning so that a high grade layer integrity in height direction as well as that of good optical coverage quality of the outer fibrous suspension layers is achieved in a fibrous web produced by said sheet forming system. In this context, producing a fibrous web, in particular one having a base weight in the range of 20 to 60 g/m2 per fibrous suspension layer at a production speed in excess of 900 m/min. should be possible.